In monolithic silicon integrated circuit (IC) devices the individual circuit chips are mounted in a suitable housing which includes a plurality of connection leads that are connected to an array of bonding pads on the IC chip. Typically these bonding pads are located adjacent to the chip edges in a peripheral array. Some form of connection lead makes contact with each bonding pad. The pad is made large enough to accomodate the lead so that the mechanical tolerances of lead location will not exceed pad size.
In silicon IC devices the typical metallization is aluminum which is commonly overcoated with a passivation material such as vapor deposited silicon dioxide (vapox) or silicon nitride or polymide plastic. In many cases this vapox contains a substantial percentage of phosporous oxide which enhances its protective capability. Such a glass is called phospho-silicate glass (PSG). This passivation is quite effective and will protect the IC from moisture, metal particles, organics and other contaminants that can deleteriously affect the device. It has been found that the aluminum metallization can be attacked electrolytically by moisture that seeps into the final package in minute quantities.
The passivation mentioned above will prevent such action but, in order to contact the IC, the passivation must be removed over the contact area of the bonding pads. Thus the aluminum bonding pads must be exposed. This means that moisture can penetrate the bonding pad area and proceed to pass underneath the passivation layer thus circumventing the passivation.
It would be desirable to passivate the bonding pads so that even if moisture is present it will not attack the pad metal.